The present invention relates to a hydrotrope and to a method for using an aqueous hydrotrope solution to remove at least a portion of a solvent from a stream of air containing the solvent. More particularly, the present invention relates to a variety of hydrotropes and to methods of using aqueous solutions of hydrotropes to reduce solvent-vapor emissions at paint spray booths.
Large quantities of coatings such as paint, lacquers and varnishes are used in the automobile, appliance, and other industries to coat finished products. A vast majority of these coatings are solvent based. Many times such coatings are applied to workpieces in enclosed areas called paint spray booths.
In paint spray booths, substantial quantities of air (in the form of air streams) are used to clean the booths of paint or lacquer particles, volatile organic paint carriers, solvents, and the like. Such air streams pick up substantial quantities of paint particles, volatile solvents or carrier vapors which are then disposed of. In the past, this disposal has consisted of discharging the vapors or particles to the atmosphere. Such a discharge of volatile organic solvent vapors presents an environmental pollution problem, as well as a serious economic problem, because the lost materials must be replaced.
Although the air streams in paint spray booths have been conventionally washed with water, water acts as a poor solvent for the volatile organic material and performs inadequately in containing these materials. As particulate material (in the form of over-sprayed paint solids) becomes trapped by the water, the paint solids tend to hold their associated solvent or carrier. This additionally results in organic solvent vapors being discharged to the atmosphere.
Current economics dictate that modification of existing paint spray booth systems to eliminate discharge of volatilized solvents into the atmosphere is expensive, often prohibitively so, using state-of-the-art techniques such as carbon adsorption, incineration and the like. Accordingly, it has been discovered that by adding certain hydrotropic substances to the wash water of paint spray booth systems, substantial quantities of solvents can be recovered.
The term "hydrotrope" here refers to a substance which has the property of increasing the aqueous solubility of a variety of otherwise only slightly water-soluble organic chemicals. But more specifically, the hydrotrope of the present invention is a chemical substance which includes an organic group chemically bonded to a polar group. The organic group can be an aromatic, an aliphatic, or combinations thereof. When the organic group is an aliphatic, the carbon chain length can range from about C.sub.3 to about C.sub.10. When the aliphatic group has a chain length beyond C.sub.10, the chemical substance acts as a surfactant. When such chemical substance is added to water, foaming is experienced. In the operations of paint spray booths, such foaming is undesirable.
When the organic group is an aromatic or a combination of aromatic and aliphatic, the chain length can range from about C.sub.6 (as when benzene is the aromatic) to about C.sub.9. Such chain length designation includes the aromatic group and the additional hydrocarbon group or groups chemically bonded thereto. Here again, the chemical substance acts as a surfactant beyond the C.sub.9 chain length range.
The polar group of the hydrotrope can be a carboxylate, a phosphate, a phosphonate, a sulfate or a sulfonate, all of which are in the form of a salt. The above-described polar groups are salts of sodium (Na), ammonium, cesium, potassium or any such salt which is highly water-soluble. The sodium, ammonium, cesium and potassium, of course, are the cationic portions of these salts. Such a salt, when chemically bonded to any one of the above-described organic groups, presents the chemical substance with surprising qualities. One quality is that the chemical substance, when added to water, greatly increases the water solubility of certain organic compounds, such as hydrocarbon-based solvents. Another quality is that the chemical substance itself is highly soluble in water. The chemical substances of the present invention perform as hydrotropic substances.
The hydrotropes of the present invention have a minimum carbon chain length restriction because if the organic group does not have an adequate number of carbon atoms, the chemical substance will not be effective in increasing the water solubility of organic solvents which are not normally water soluble. The key here is that the chemical substance have enough of an "organic character". Having such, the chemical substance can then cause such an organic solvent to become dissolved in water. Thus, a minimum carbon chain length restriction, as to the organic group, provides the chemical substance with this desired quality.
Particularly useful hydrotropes are sodium hexanoate, sodium octanoate, sodium xylenesulfonate, and sodium cumenesulfonate. Sodium hexanoate is a sodium salt of hexanoic acid. Sodium octanoate is a sodium salt of octanoic acid.
The sodium xylenesulfonate described here for purposes of teaching the present invention is available commercially from Witco Chemical Corporation of New York, N.Y. The sodium cumenesulfonate described here for purposes of teaching the present invention is a commercially available sulfonated cumene. As such, it is normally sulfonated in the para position, but may include some cumene sulfoanted in the meta or ortho position. The sodium cumenesulfonate described here contains cumene which was sulfonated primarily in the para position.